Exchange functions of this kind are widely used in the field of medicine. An example of such functions is found in blood oxygenators used for supplying oxygen (O.sub.2) to and removing carbon dioxide (CO.sub.2) from blood in so-called heart-lung machines. Another example is found in so-called dialysis filters for removing liquid and waste salts from the blood in artificial kidneys.
In exchangers of this kind the two fluids between which an exchange is to take place are separated by a membrane which is of a nature such as to enable the exchange desired to be effected therethrough. In the case of one large, principal group of such exchangers, the membrane consists of a very large number of capillary fibres, i.e. hollow fibres, through which one of the fluids passes while the other of said fluids flows around the outer surfaces of the fibres, the walls of the fibres having a nature such as to permit the desired exchange to take place. For example, the fibre walls of a blood oxygenator are permeable to gas, whereas the fibre walls of a dialysis filter are permeable to those salt ions which are to be removed from the blood. For instance, the nature of the fibre wall may be such as to allow liquid to pass therethrough under the influence of a difference in pressure across the fibre wall. The nature of the fibre wall is thus contingent upon or determined by the exchange desired between the two fluids, while the internal diameters of the fibres and the wall thicknesses are contingent on the fluid which is intended to flow through the fibres.
Several important requirements are placed on exchangers which operate with such capillary filter membranes and which are intended for use in medicinal techniques for treating primarily blood.
Thus, the total volume of blood in the apparatus should be the smallest possible;
The passages through or along which the blood travels shall be formed so that no eddy currencies or like turbulence flow will occur and so that no locations are found where blood could stand stationary and coagulate; The flow rate of the fluid which flows around the fibres should be as uniform as possible throughout the entire volume occupied by the fibre bundle.
Hitherto known exchangers provided with capillary fibres do not fulfill all of these requirements in a satisfactory manner.